U.S. patent application number 13/123846 was filed with the patent office on 2012-06-28 for microporous membrane winding and method for manufacturing the same.
Invention is credited to Shintaro Inaba, Daisuke Inagaki, Hisashi Takeda.
Application Number | 20120164538 13/123846 |
Document ID | / |
Family ID | 43627949 |
Filed Date | 2012-06-28 |
United States Patent
Application |
20120164538 |
Kind Code |
A1 |
Inagaki; Daisuke ; et
al. |
June 28, 2012 |
MICROPOROUS MEMBRANE WINDING AND METHOD FOR MANUFACTURING THE
SAME
Abstract
A microporous membrane winding includes a microporous membrane
wound around a core. The core has an outer diameter of 5 inches or
greater, and has an outer surface with a surface roughness of 3.0
.mu.m or less. A microporous membrane that is excellent in
thickness uniformity and is favorably used as a separator for a
lithium-ion secondary battery can be obtained from the microporous
membrane winding.
Inventors: |
Inagaki; Daisuke;
(Chiyoda-ku, JP) ; Takeda; Hisashi; (Chiyoda-ku,
JP) ; Inaba; Shintaro; (Chiyoda-ku, JP) |
Family ID: |
43627949 |
Appl. No.: |
13/123846 |
Filed: |
August 25, 2010 |
PCT Filed: |
August 25, 2010 |
PCT NO: |
PCT/JP2010/064359 |
371 Date: |
July 25, 2011 |
Current U.S.
Class: |
429/249 ;
242/159; 242/520 |
Current CPC
Class: |
H01M 50/403 20210101;
H01M 50/449 20210101; H01M 50/411 20210101; Y02E 60/10 20130101;
H01M 50/446 20210101; H01M 50/463 20210101; Y02E 60/13 20130101;
H01M 10/0525 20130101; H01G 11/52 20130101 |
Class at
Publication: |
429/249 ;
242/159; 242/520 |
International
Class: |
H01M 2/16 20060101
H01M002/16; B65H 18/08 20060101 B65H018/08; B65H 18/28 20060101
B65H018/28 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 2009 |
JP |
2009-194445 |
Claims
1-23. (canceled)
24. A microporous membrane winding comprising a core and a
microporous membrane wound around the core, wherein the core has an
outer diameter of 5 inches or greater, and has an outer surface
with a surface roughness of 3.0 .mu.m or less.
25. The microporous membrane winding according to claim 24, wherein
the outer surface of the core has a root mean square value
roughness of 3.0 .mu.m or less.
26. The microporous membrane winding according to claim 24 or 25,
wherein the outer surface of the core has an average length of
curvilinear elements of 300 .mu.m or less.
27. The microporous membrane winding according to claim 24 or 25,
wherein the core has an absolute value of a thermal expansion
coefficient of 150.times.10.sup.-6/K or less.
28. The microporous membrane winding according to claim 24 or 25,
wherein the core has a swelling rate of 0.06% or less.
29. The microporous membrane winding according to claim 24 or 25,
wherein a value obtained by dividing the number of laminations
(times) of the microporous membrane winding by its winding length
(m) is 2.0 or less.
30. The microporous membrane winding according to claim 24 or 25,
wherein the core has a maximum backlash of 0.30 mm or less, the
maximum backlash being measured on a surface plate horizontally
installed having a face larger than a side face of the core
perpendicular to a rotating shaft thereof.
31. The microporous membrane winding according to claim 24 or 25,
wherein both of a MD tensile elastic modulus and a TD tensile
elastic modulus of the microporous membrane are in the range of
from 10 to 120 N/cm.
32. The microporous membrane winding according to claim 24 or 25,
wherein the core has an outer diameter in the range of from 5 to 15
inches.
33. The microporous membrane winding according to claim 24 or 25,
wherein the microporous membrane is composed of polyolefin, the
polyolefin comprising at least polyethylene with weight average
molecular weight of 500,000 or less.
34. The microporous membrane winding according to claim 24 or 25,
wherein the microporous membrane is composed of polyethylene with
weight average molecular weight of 500,000 or less.
35. A lithium-ion secondary battery comprising a microporous
membrane obtained from the microporous membrane winding according
to claim 24 or 25.
36. A method for manufacturing a microporous membrane winding,
comprising the steps of: preparing a core with an outer diameter of
5 inches or greater and with an outer surface having a surface
roughness of 3.0 .mu.m or less; preparing a microporous membrane;
and winding the microporous membrane around the core.
37. The method for manufacturing a microporous membrane winding
according to claim 36, wherein the outer surface of the core has a
root mean square value roughness of 3.0 .mu.m or less.
38. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein the outer surface of the core
has an average length of curvilinear elements of 300 .mu.m or
less.
39. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein the core has an absolute value
of a thermal expansion coefficient of 150.times.10.sup.-6/K or
less.
40. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein the core has a swelling rate
of 0.06% or less.
41. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein a value obtained by dividing
the number of laminations (times) of the microporous membrane
winding by its winding length (m) is 2.0 or less.
42. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein the core has a maximum
backlash of 0.30 mm or less, the maximum backlash being measured on
a surface plate horizontally installed having a face larger than a
side face of the core perpendicular to a rotating shaft
thereof.
43. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein both of a MD tensile modulus
and a TD tensile modulus of the microporous membrane are in the
range of from 10 to 120 N/cm.
44. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein the core has an outer diameter
in the range of from 5 to 15 inches.
45. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein the microporous membrane is
composed of polyolefin, the polyolefin comprising at least
polyethylene with weight average molecular weight of 500,000 or
less.
46. The method for manufacturing a microporous membrane winding
according to claim 36 or 37, wherein the microporous membrane is
composed of polyethylene with weight average molecular weight of
500,000 or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a microporous membrane
winding favorably used for a lithium-ion secondary battery and a
lithium-ion secondary battery using a microporous membrane obtained
(unwound) from the microporous membrane winding, and relates to a
method for manufacturing the same.
BACKGROUND ART
[0002] Microporous membranes are widely used as membranes for
separation or permselective separation of various substances and as
materials for isolation of various substances. Examples of the use
of the membranes may include microfiltration membranes, fuel cell
separators, capacitor separators, base materials for functional
membranes to allow a novel function to develop by filling pores
with a functional material, and battery separators. Above all,
microporous polyolefin membranes are preferable as separators for
lithium-ion batteries widely used in laptop personal computers,
cell phones, and digital cameras, for example.
[0003] For instance, Patent Document 1 discloses a microporous
membrane with a small surface roughness. Patent Document 2
discloses a polyester film wound around a core with a small surface
roughness. Patent Document 3 discloses a technique enabling a film
property to be maintained during winding while preventing
misalignment during winding by improving the mechanical strength
and the elastic modulus of a wound film.
PRIOR ART DOCUMENTS
Patent Documents
[0004] Patent Document 1: JP2009-91461 A [0005] Patent Document 2:
JP09-272148 A [0006] Patent Document 3: JP10-340715 A
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0007] In recent years, as batteries have higher capacity,
membranes as separators used in the batteries especially have been
made thinner and have been required to have more uniform membrane
thickness. In general, as separators for high-capacity lithium-ion
secondary batteries, a thin membrane is used because electrode can
be used as much as possible.
[0008] Conventional separators, however, still have room for
improvement in the uniformity of membrane thickness.
[0009] It is an object of the present invention to provide a
microporous membrane winding providing a microporous membrane that
is preferable as a separator for a lithium-ion secondary battery
and has excellent thickness uniformity in especially MD (machine
direction of an extruder when a microporous membrane is formed that
is in parallel with a resin discharging direction).
Means for Solving the Problem
[0010] In order to improve the thickness uniformity of a
microporous membrane, the present inventors have found that
attention needs to be paid to a shape, properties (shape of the
core surface, thermal expansion coefficient, swelling rate due to
humidity) and a quality of a core around which a microporous
membrane is wound.
[0011] More specifically, when a microporous membrane is wound
around a core, a phenomenon called "constriction due to winding"
occurs to the membrane because it contains micropores. Then, the
constriction due to winding will cause transferring the surface
shape of the core onto the microporous membrane, leading to
tendency of generating deformation in the microporous membrane.
[0012] Such deformation propagated over the entire winding may lead
to significant thickness nonuniformity especially in the comparison
between a portion of the microporous membrane located inside the
winding and a portion located outside, thus may degrade the quality
of a battery. Such tendency becomes more serious with decrease in
membrane thickness or increase in winding-around length (this may
be described as "winding length").
[0013] When a core of a microporous membrane winding swells or
shrinks due to a change in temperature or humidity during
transportation, for example, the above deformation of the
microporous membrane tends to grow.
[0014] Further, when a core is attached to an individual or a
coaxial-armed reeler or winder, a side face of the core is pressed
against an alignment face of such as a winding arm or a wall
surface of the winder to set an attachment position of the core. As
a result of extensive investigation, the present inventors have
found that an installation angle to the alignment face does not
become constant because of a difference in force to press the core
against the alignment face in fixing the core, and so a microporous
membrane is wound around the core while the core being fixed on a
slight tilt, which leads to the appearance of winding misalignment
or wrinkles in unwinding of the microporous membrane with a battery
winder.
[0015] As a result of extensive investigation to achieve the
above-stated object, the present inventors have found that the
above-stated problems can be solved by winding a microporous
membrane around a core with a specific outer diameter and a
specific surface roughness, thus accomplishing the present
invention.
[0016] The present inventors further have found that when a root
mean square value roughness and an average length of curvilinear
elements on an outer surface of a core, a thermal expansion
coefficient and a swelling rate of the core, backlash and a
relationship between the number of laminations of the winding and
the winding length of a microporous membrane are within a specific
range, the thickness uniformity of the microporous membrane unwound
from the microporous membrane winding can be further improved.
[0017] That is, one aspect of the present invention is as
follows:
[0018] a microporous membrane winding includes a core and a
microporous membrane wound around the core, the core has an outer
diameter of 5 inches or greater, and has an outer surface with a
surface roughness of 3.0 .mu.m or less.
[0019] Another aspect of the present invention is as follows:
[0020] a method for manufacturing a microporous membrane winding,
includes the steps of: preparing a core with an outer diameter of 5
inches or greater and with an outer surface having a surface
roughness of 3.0 .mu.m or less;
[0021] preparing a microporous membrane; and
[0022] winding the microporous membrane around the core.
Effect of the Invention
[0023] According to the present invention, a microporous membrane
winding can be achieved, that is capable of providing a microporous
membrane with favorable uniformity in thickness distribution
(membrane thickness uniformity).
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 schematically illustrates an apparatus to measure a
maximum backlash.
MODE FOR CARRYING OUT THE INVENTION
[0025] The following describes best mode for carrying out the
present invention (hereinafter abbreviated as "present embodiment")
in detail. The present invention is not limited to the embodiments
below, and can be embodied in various manners within the scope of
the gist of the invention.
[0026] A microporous membrane winding of the present embodiment is
a microporous membrane winding obtained by winding a microporous
membrane around a core, wherein the core has an outer diameter of 5
inches or greater, and an outer surface of the core has a surface
roughness of 3.0 .mu.m or less. Such a configuration allows a
microporous membrane obtained from the microporous membrane winding
in the present embodiment to have favorable membrane thickness
uniformity as required especially for a high-capacity lithium-ion
secondary battery.
[0027] In the present embodiment, "1 inch" can be converted into
25.4 mm.
[0028] Herein, a "winding" refers to a microporous membrane with a
uniform width and a predetermined length that is wound around a
core. The winding length and the width are not limited especially,
and typically the width is in the range of from 50 m to 10,000 m,
and the width is in the range of from a few mm to 1,000 mm. When
the microporous membrane is used as a separator for a lithium-ion
secondary battery, the winding length is typically in the range of
from 500 m to 5,000 m, and the width is typically in the range of
from 20 mm to 500 mm.
[0029] The "core" refers to a winding core having a circular
cylindrical shape in outline, including a paper core and a
cylindrical winding core made of ABS resin or phenol resin, used
for winding of a microporous membrane. In order to reduce the
constriction of the wound microporous membrane due to winding, such
a core has an outer diameter of 5 inches or greater, preferably of
6 inches or greater, more preferably of 8 inches or greater, and
still more preferably of 9 inches or greater. The upper limit of
the outer diameter of the core, but not limited to, is preferably
of 20 inches or less and more preferably of 15 inches or less from
the standpoint of handling.
[0030] The width (length) of the core is typically in the range of
from a few mm to 1,000 mm. However, since the effects of the
present invention is benefitical for a wider core, the width
preferably is 10 mm or greater and 1,000 mm or less, more
preferably 50 mm or greater and 1,000 mm or less, and particularly
preferably 100 mm or greater and 1,000 mm or less. This is because
a wider winding is susceptible to quality of its core.
[0031] The "surface roughness" refers to a so-called Ra (arithmetic
average roughness).
[0032] The surface roughness Ra of the outer surface of the core is
3.0 .mu.m or less, preferably is 2.0 .mu.m or less, more preferably
is 1.0 .mu.m or less, still more preferably is 0.8 .mu.m or less,
particularly preferably is 0.5 .mu.m or less, and most preferably
is 0.3 .mu.m or less. The surface roughness of the outer surface of
3.0 .mu.m or less can reduce the transferring of unevenness of the
core onto the microporous membrane near the innermost layer when
the microporous membrane is wound tightly. Thereby, the membrane
thickness uniformity of the microporous membrane between the
innermost layer and the outer layer of the winding can be improved
even when the microporous membrane is thin. The improved membrane
thickness uniformity can reduce variation in battery capacity.
[0033] The lower limit of the surface roughness, but no limited to,
is preferably 0.01 .mu.m or greater, more preferably is 0.05 .mu.m
or greater, and still more preferably is 0.1 .mu.m or greater.
[0034] Similarly to Ra, the root mean square value roughness Rq of
the outer surface of the core also preferably is 3.0 .mu.m or less,
more preferably is 2.0 .mu.m or less, still more preferably is 1.0
.mu.m or less, still more preferably is 0.8 .mu.m or less,
particularly preferably is 0.5 .mu.m or less, and most preferably
is 0.3 .mu.m or less. Rq of the outer surface of 3.0 .mu.m or less
can reduce the transferring of unevenness of the core onto the
microporous membrane near the innermost layer when the microporous
membrane is wound tightly. Thereby, the membrane thickness
uniformity of the microporous membrane between the innermost layer
and the outer layer of the winding can be improved even when the
microporous membrane is thin. The improved membrane thickness
uniformity can reduce variation in battery capacity.
[0035] The lower limit of Rq, but no limited to, is preferably 0.01
.mu.m or greater, more preferably is 0.05 .mu.m or greater, and
still more preferably is 0.1 .mu.m or greater.
[0036] The average length Sm of the curvilinear elements on the
outer surface of the core preferably is 300 .mu.m or less, more
preferably is 200 .mu.m or less and still more preferably is 100
.mu.m or less. The Sm of 300 .mu.m or less leads to improved
membrane thickness uniformity of the microporous membrane for the
same reason as in Ra and Rq. The lower limit of Sm, but no limited
to, is preferably 0.1 .mu.m or greater and more preferably is 1
.mu.m or greater.
[0037] A method for obtaining Ra, Rq and Sm in the above-stated
ranges is not limited especially, and Ra, Rq and Sm can be made
desired values by cutting, polishing or grinding the outer surface
of the core precisely, or by brining the outer surface of the core
into contact with a heated mirror-finished roll.
[0038] When the core is made of paper, for example, a resin layer
is applied on the outer surface of the core, and the resin layer is
cut, polished or ground, or is brought into contact with a heated
mirror-finished roll, whereby a desired surface roughness can be
obtained.
[0039] The "outer surface" refers to a portion of the surface
around which the microporous membrane is wound, and a portion of
the surface of the core with which the microporous membrane does
not contact is not limited especially.
[0040] Since the effects of the present embodiment can be obtained
even when a portion with the above-mentioned surface roughness
exists partially, a ratio of the region having the above-mentioned
surface roughness to the outer surface is not limited especially.
The ratio of the region having a surface roughness of 3.0 .mu.m or
less to the entire outer surface (a portion of the core surface
that is brought into contact with the microporous membrane) is
preferably 80% or greater, more preferably 90% or greater, and
still more preferably 100%.
[0041] An average of the surface roughness of the overall length of
the core in the width direction is preferably 3.0 .mu.m or
less.
[0042] A preferred material of the core, but not limited to,
includes plastic and a thermosetting resin etc, because they have a
small thermal expansion coefficient, improved stiffness, a small
swelling rate for humidity, and excellent winding property. Note
that when the core is made of paper, its surface may be coated with
a resin etc, whereby a desired property can be easily obtained.
[0043] An absolute value of the thermal expansion coefficient is
preferably 150.times.10.sup.-6/K or less, more preferably
100.times.10.sup.-6/K or less, and particularly preferably
50.times.10.sup.-6/K or less. For example, in transporting the
microporous membrane winding for a long time, if the absolute value
of the thermal expansion coefficient is 150.times.10.sup.-6/K or
less, the tendency of the core to swell or shrink under the
influence of ambient temperatures is reduced, thus the microporous
membrane is not subjected to a pressure and its quality can be
maintained. As a result, variations in battery capacity also can be
reduced effectively.
[0044] The swelling rate for humidity can be evaluated based on a
value (swelling rate) that is calculated from the outer diameter
when the core is left to stand for 24 hours at the temperature of
25.degree. C. and the relative humidity of 50% and under
humidification of 100%. The swelling rate is preferably 0.06% or
less, more preferably 0.04% or less and particularly preferably
0.02% or less. Such a core can be obtained by selecting a material
appropriately or optimizing an aging condition. A material suitable
for manufacturing a core with a low swelling rate for humidity
includes polytetrafluoroethylene, polyethylene, ABS with reduced
polar groups and heat-treated Bakelite etc.
[0045] For example, in transporting the microporous membrane
winding for a long time, if the swelling rate is 0.06% or less, the
tendency of the core to swell under the influence of ambient
humidity is reduced, thus the microporous membrane is not subjected
to a pressure in the thickness direction, and its quality can be
easily maintained. As a result, variations in battery capacity also
can be effectively reduced.
[0046] The number of laminations (the number of windings) (times)
of the microporous membrane with respect to the winding length (m)
of the microporous membrane (overall length of the wound
microporous membrane) (the number of laminations/winding length) in
the microporous membrane is preferably 2.0 (times/m) or less, more
preferably of 1.5 or less, and particularly preferably of 1.0 or
less.
[0047] When this value is small, the number of laminations of the
microporous membrane with respect to the winding length is small,
and if this value is 2.0 or less, the constriction due to winding
of the microporous membrane can be reduced, and therefore the
membrane thickness uniformity of the microporous membrane unwound
from the winding can be improved. In addition, as compared with the
microporous membrane with the same thickness and winding length,
the outer diameter value of the winding becomes relatively large,
and therefore R (radius of the outer diameter value of the winding)
becomes large, so that frequency of rubbing between microporous
membranes can be reduced during winding, which leads to the
reduction in static electricity in the winding. A reduced static
electricity can reduce the appearance of wrinkles in pulling out
the microporous membrane from the microporous membrane winding to
prepare a wound battery, and therefore a wound battery property can
be improved.
[0048] The static electricity of the winding at this time is
preferably 1.0 kV or less for the above-stated reason, more
preferably 0.6 kV or less, still more preferably 0.4 kV or less,
and most preferably 0.2 kV or less.
[0049] The core has a maximum backlash, which is measured on a
surface plate horizontally installed having a face larger than a
side face of the core perpendicular to the rotating shaft, of
preferably 0.30 mm or less, more preferably 0.20 mm or less, still
more preferably 0.10 mm or less, and most preferably 0.05 mm or
less.
[0050] This backlash refers to a displacement of the outermost
circumference of the core when the core is placed on a surface
plate horizontally installed having a face larger than a side face
of the core perpendicular to the rotating shaft in such a manner
that the side faces of the core perpendicular to the core rotating
shaft face upward and downward (one of the side faces perpendicular
to the core rotating shaft comes into contact with the surface
plate), and a load of 1 kg is applied to any point on the outermost
circumference of the core.
[0051] The maximum backlash refers to the largest value among the
values of backlash that are measured when such measurement is
performed with respect to 32 points on the outermost circumference
of the two side faces of the core perpendicular to the core
rotating shaft (16 points for each face).
[0052] If the maximum backlash of 0.30 mm or less, a slight
inclination or displacement from a fixed position in fixing the
core by pressing its side face against a core alignment face of
such as a reeler or a winder can be reduced. Thereby, the
appearance of wrinkles or misalignment in winding the microporous
membrane around the core, or the generation of hunting motion or
fluttering in unwinding the microporous membrane from the winding
can be reduced. Such defect reduction leads to decrease in
defective rate of products having the wound microporous
membrane.
[0053] The lower limit of backlash is not limited especially, but a
core free from backlash (maximum backlash of 0 mm) can lead to a
more remarkable effect.
[0054] A method for making the core with a maximum backlash within
such a range is not limited especially, and a core with a desired
maximum backlash can be obtained by using a mold with high
dimension accuracy or by polishing the side face of the core
perpendicular to the rotating shaft precisely, for example.
[0055] The porosity of the microporous membrane wound around the
core is preferably 20% or greater and more preferably 30% or
greater from the standpoint of allowing the microporous membrane to
follow up the rapid movement of lithium ions, whereas is preferably
90% or less, more preferably 80% or less, and still more preferably
50% or less from the standpoint of the membrane strength and self
discharge.
[0056] The air permeability of the microporous membrane wound
around the core is preferably 1 sec or greater, and more preferably
50 sec or greater from the standpoint of a balance between
thickness, porosity and average porous diameter. On the other hand,
it is preferably 400 sec or less and more preferably 300 sec or
less from the standpoint of permeability.
[0057] The tensile strength of the microporous membrane wound
around the core is preferably 10 MPa or greater and more preferably
30 MPa or greater in both directions of MD and TD (direction
perpendicular to MD, membrane width direction). The tensile
strength of 10 MPa or greater is preferred from the standpoint of
reducing slit or rupture during winding a battery, from the
standpoint of reducing short-circuit caused by foreign matters in
the battery, or from the standpoint of reducing pattern
transferring from a core with a high surface roughness. The upper
limit of the tensile strength is not limited especially, and is
preferably 500 MPa or less, more preferably 300 MPa or less and
still more preferably 200 MPa or less from the standpoint of
relaxing the microporous membrane at an early stage during heat
test to weaken the contractile force, resulting in improved
safety.
[0058] The tensile elastic modulus of the microporous membrane
wound around the core is preferably 120 N/cm or less in both
directions of MD and TD. If the tensile elastic modulus is 120 N/cm
or less, the membrane is not excessively oriented as a separator
for lithium-ion secondary battery, and a shutdown material such as
polyethylene generate stress relaxation at an early stage when it
melts and shrinks during heat test etc. Thereby, shrinking of a
separator in a battery can be suppressed, and so that short-circuit
between electrodes can be prevented (can improve safety of the
separator during heating). The tensile elastic modulus is more
preferably 100 N/cm or less, and still more preferably 90 N/cm or
less. The lower limit, but not limited to, is preferably 10 N/cm or
greater, more preferably 30 N/cm or greater, and still more
preferably 50 N/cm or greater. A microporous membrane with such low
tensile elastic modulus can be easily achieved by including
polyethylene with weight average molecular weight of 500,000 or
less in polyolefin making up the microporous membrane.
[0059] A microporous membrane with such low tensile elastic modulus
has a tendency to form unevenness especially at an innermost
portion due to the contraction when it is wound. However, if the
winding is prepared in combination with the above-stated core, a
microporous membrane winding having both of favorable safety and
membrane thickness uniformity can be achieved.
[0060] The tensile elastic modulus of the microporous membrane can
be adjusted appropriately by adjusting the degree of stretching or
by performing relaxing following the stretching as needed, for
example.
[0061] The thickness of the microporous membrane wound around the
core, but not limited to, is preferably 1 .mu.m or greater from the
standpoint of membrane strength, and is preferably 500 .mu.m or
less and more preferably 100 .mu.m or less from the standpoint of
permeability. From the standpoint of the use in a lithium-ion
second battery with relatively high capacity in recent years, the
thickness is preferably 25 .mu.m or less, more preferably 20 .mu.m
or less, still more preferably 16 .mu.m or less and particularly
preferably 12 .mu.m or less.
[0062] The core of the present embodiment with a specific outer
diameter and a specific surface roughness shows more remarkable
effects in combination with a thin microporous membrane especially.
This may be because influences of the surface roughness of the core
increase as a thickness of the microporous membrane with respect to
the surface roughness of the core decrease.
[0063] The above-mentioned ranges of winding length, porosity, air
permeability, tensile strength, tensile elastic modulus, and
membrane thickness are preferred from the standpoint achieving a
microporous membrane winding capable of providing a microporous
membrane with favorable membrane thickness uniformity in
combination with the above-stated specific core.
[0064] The above-mentioned various properties of the "wound-around
microporous membrane" are values determined from the measurement
for an outermost layer of the microporous membrane wound around the
core.
[0065] The microporous membrane may be a single-layer or in a
laminated product.
[0066] A method for manufacturing a microporous membrane winding
according to the present embodiment will be described below.
However, the polymer type, the solvent type, the extrusion method,
the stretching method, the extraction method, the pore formation
method, the heat setting method (called heat treatment as well) and
the like are just examples.
[0067] Firstly, in a method for manufacturing a microporous
membrane winding in the present embodiment, a method for preparing
a microporous membrane (manufacturing method for a microporous
membrane) is not limited especially. For instance, the method
preferably includes a step of melting, kneading and extruding a
polymer material and a plasticizer, or a polymer material, a
plasticizer and an inorganic agent, a step of stretching; a step of
extracting plasticizer (and an inorganic agent as needed); and step
of heat setting (called heat treatment as well). Alternatively, the
microporous membrane may be manufactured by stretching the film
crystallized appropriately to form pores without using a solvent,
or by streching a kneaded product of inorganic filler or organic
filler and polymer material to form pores in an interface between
the polymer and the fillers. Further, an inorganic agent may be
coated on the surface of the microporous membrane.
[0068] A preferred embodiment of the microporous membrane is
prepared by the above-mentioned method.
[0069] More specifically, the method for manufacturing a
microporous membrane winding may include the following steps of (a)
to (e):
[0070] (a) a kneading step of kneading a polyolefin composition
containing polyolefin, a plasticizer, and an inorganic agent as
needed;
[0071] (b) a sheet shaping step of extruding the resultant of
kneading after the kneading step to shape it into a sheet (this may
be single-layer or a laminated product), followed by solidification
by cooling;
[0072] (c) a stretching step of extracting the plasticizer and the
inorganic agent as needed after the sheet shaping step, and further
stretching the sheet in at least a monoaxial direction;
[0073] (d) a post-processing step of extracting the plasticizer and
the inorganic agent as needed after the stretching step, and
further performing heat treatment; and
[0074] (e) a step of slitting the obtained microporous membrane as
needed and winding the resultant around a predetermined core.
[0075] [(a) Step]
[0076] The polyolefin used in the above (a) step includes a
homopolymer of ethylene or propylene, or a copolymer formed with at
least two monomers selected from the group consisting of ethylene,
propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and
norbornene. This may be a mixture thereof.
[0077] Comprising polyolefin with weight average molecular weight
of 500,000 or less (comprising preferably 40% by mass or greater
and more preferably 80% by mass or greater with respect to
polyolefin as a whole) is preferable because shrinkage of polymer
can be relaxed at an early stage during heat test, for example, and
safety can be easily maintained during heat safety test. In the
case of using polyolefin with weight average molecular weight of
500,000 or less, however, the microporous membrane obtained has
tendency to have a reduced elastic modulus in the thickness
direction as compared with the case where polyolefin with weight
average molecular weight exceeding 500,000 is used, and thus has a
tendency to receive a transferring of unevenness of the core
thereon. In this respect, the present inventors have found that
using the above-mentioned specific core brings a surprising effect
of maintaining safety while suppressing variations in battery
quality even when the microporous membrane is composed of
polyolefin with weight average molecular weight of 500,000 or less.
This effect is more remarkable when polyolefin with weight average
molecular weight of 500,000 or less only is used as the polyolefin
forming the microporous membrane.
[0078] When polyethylene is used as the polyolefin, high-density
polyethylene (homopolymer) is preferably used from the standpoint
of enabling heat setting at a higher temperature without closing
pores. However, low-density polyethylene also may be used. The
weight average molecular weight of the microporous membrane as a
whole is preferably 100,000 or greater and 1,200,000 or less, and
is more preferably 150,000 or greater and 800,000 or less. The
weight average molecular weight of 100,000 or greater is preferred,
since it gives resistance against membrane rupture during melting
to the membrane, and the weight average molecular weight of
1,200,000 or less is preferred, since it facilitates the extruding
step, and speeds up the relaxation of a contractile force during
melting, thus improves heat resistance of the membrane.
[0079] In the above-mentioned (a) step, when a polymer other than
polyethylene is blended, the ratio of the polymer other than
polyethylene to the total amount of polymer is preferably 1 to 80%
by mass, more preferably 2 to 50% by mass, still more preferably 3
to 20% by mass and particularly preferably 5 to 10% by mass. If the
ratio of the polymer other than polyethylene is 1% by mass or
greater, compressive resistance in the thickness direction improves
when the polymer has an elastic modulus higher than that of
polyethylene, and heat resistance improves, when the polymer has a
melting point higher than that of polyethylene.
[0080] If the ratio of the polymer other than polyethylene is 80%
by weight or less, it becomes easy to secure the permeability due
to uniformity with polyethylene.
[0081] Well-known additives can be mixed to the polyolefin
composition used in the above-mentioned (a) step, including metal
soaps such as calcium stearate and zinc stearate, ultraviolet
absorbers, light stabilizers; antistatic agents, anti-fogging
agents, coloring pigments and the like.
[0082] Examples of the plasticizer may include an organic compound
that can form a uniform solution with polyolefin at its boiling
point or less. Specific examples include decalin, xylene, dioctyl
phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl
alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, and
paraffin oil. Among them, paraffin oil and dioctyl phthalate are
preferably used, and they may be used in combination of two or
more.
[0083] The ratio of the plasticizer is not limited especially, and
20% by mass or greater is preferred based on the total mass of
polyolefin, plasticizer and an inorganic agent mixed as needed from
the standpoint of the porosity of the microporous membrane
obtained, and 90% by mass or less is preferred from the standpoint
of viscosity during melting and kneading.
[0084] Examples of the inorganic agent include oxide ceramics such
as alumina, silica (silicon oxide), titania, zirconia, magnesia,
ceria, yttria, zinc oxide and iron oxide, nitride ceramics such as
silicon nitride, titanium nitride and boron nitride, ceramics such
as silicon carbide, calcium carbonate, aluminum sulfate, aluminum
hydroxide, potassium titanate, talc, kaolin clay, kaolinite,
halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite,
bentonite, asbestos, zeolite, calcium silicate, magnesium silicate,
diatomaceous earth and silica sand, and glass fibers. They can be
used alone or in combination of two or more. From the standpoint of
electrochemical stability and improved heat resistance, silica,
alumina and titania are preferably used among them.
[0085] The melting may be performed in the following manner. First,
a part or all of the raw materials are mixed in advance with a
Henschel mixer, a ribbon blender, a tumbler blender or the like as
needed. Then, all of the raw materials are melted and kneaded with
a screw extruder such as single screw extruder or twin screw
extruder, a kneader, a mixer or the like. The kneaded resultant is
then extruded through a T die or a ring die. At this time, it may
be extruded as a single-layer or a laminated product.
[0086] Preferably, during kneading, the raw material polymer is
mixed with an antioxidant at a predetermined concentration, and
then the atmosphere is substituted with nitrogen atmosphere, where
the melting and kneading are performed while keeping the nitrogen
atmosphere. The melting and kneading are performed preferably at a
temperature of 160.degree. C. or greater and more preferably at a
temperature of 180.degree. C. or greater. On the other hand, less
than 300.degree. C. is preferred and less than 240.degree. C. is
more preferred.
[0087] [(b) Step]
[0088] Examples of method of sheet shaping include solidifying the
melt obtained by extrusion after melting and kneading by
compression cooling. Examples of the cooling method include a
method of bringing the melt into direct contact with a cooling
medium such as cool air or cooling water and a method of bringing
the melt into contact with a roll or a press cooled with a
refrigerant. The latter method of bringing the melt into contact
with a roll or press cooled with a refrigerant is preferred because
it is excellent in controlling the sheet thickness.
[0089] [(c) Step]
[0090] Examples of method of sheet stretching include MD monoaxial
stretching with a roll stretching machine, TD monoaxial stretching
with a tenter, sequential biaxial stretching with a combination of
a roll stretching machine and a tenter, sequential biaxial
stretching with a combination of a tenter and another tenter, and
simultaneous biaxial stretching using a simultaneous biaxial tenter
or blown film extrusion. The simultaneous biaxial stretching is
preferably used in order to obtain a more uniform film. The total
area magnification is preferably 8 times or greater, more
preferably 15 times or greater and still more preferably 30 times
or greater from the standpoint of a balance between uniformity of
membrane thickness, tensile elasticity, porosity and average porous
diameter. The total area magnification of 8 times or greater makes
it easy to obtain a sheet with high strength and favorable
thickness distribution.
[0091] The plasticizer or the inorganic agent may be extracted by
immersing the membrane in an extraction solvent or showering the
membrane with an extraction solvent. The extraction solvent used is
preferably a poor solvent for the polyolefin, is a good solvent for
the plasticizer and the inorganic agent, and has a boiling point
lower than the melting point of the polyolefin. Examples of such an
extraction solvent include hydrocarbons such as n-hexane and
cyclohexane, halogenated hydrocarbons such as methylene chloride,
1,1,1-trichloroethane and fluorocarbon, alcohols such as ethanol
and isopropanol, ketones such as acetone and 2-butanone and
alkaline water, which may be used alone or in combination
[0092] The inorganic agent may be extracted wholly or partially at
any step of the entire procedure or it may be left in the product.
The order, method and frequency of the extraction are not limited
especially. The extraction of the inorganic agent may be omitted as
needed.
[0093] [(d) Step]
[0094] Examples of the heat treatment method include heat setting
using a tenter or a roll stretching machine to perform stretching
and a relaxation operation. The relaxation operation refers to a
contracting operation at a predetermined temperature and relaxation
rate in the MD and/or TD of the membrane. The relaxation rate means
a value obtained by dividing the MD size of the membrane after the
relaxation operation by the MD size of the membrane before the
operation, a value obtained by dividing the TD size of the membrane
after the relaxation operation by the TD size of the membrane
before the operation, or a value obtained by multiplying the
relaxation rate of the MD by the relaxation rate of the TD when the
membrane is relaxed in both the MD and TD. The predetermined
temperature (temperature in the relaxation operation) is preferably
100.degree. C. or greater from the standpoint of thermal shrinkage
rate and preferably less than 135.degree. C. from the standpoint of
porosity and permeability. The predetermined relaxation rate is
preferably 0.9 or less, more preferably 0.8 or less from the
standpoint of thermal shrinkage rate. It is, on the other hand,
preferably 0.6 or greater from the standpoint of prevention of the
appearance of wrinkles, porosity and permeability. The relaxation
operation may be performed in both the MD and TD. However, the
thermal shrinkage can be reduced not only in the operation
direction but also a direction vertical thereto by the relaxation
operation in either one of the MD and TD.
[0095] [(e) Step]
[0096] Any special condition is not imposed on the step of, after
the formation of the microporous membrane, winding the microporous
membrane around a core (including a slitting step as needed) other
than using the core having an outer diameter of 5 inches or greater
and an outer surface with a surface roughness of 3.0 .mu.m or less.
Herein, preferred embodiments of the core are as stated above.
[0097] In addition to the above steps of (a) to (e), the method for
manufacturing a microporous membrane winding may include a step of
laminating a plurality of single-layers in order to obtain a
laminated product. The method further may include a surface
treatment step such as exposure to electron beam, exposure to
plasma, application of a surfactant, or chemical modification.
[0098] The microporous membrane obtained from the microporous
membrane winding of the present embodiment has a well-kept
thickness distribution as compared with a conventional microporous
membrane. Therefore such a microporous membrane is preferably used
as a separator for a high-capacity battery especially from the
standpoint of obtaining a uniform battery property.
[0099] The parameters mentioned above can be measured in accordance
with the measurement methods used in Examples described below
unless otherwise specified.
EXAMPLES
[0100] Next, the present embodiment will be described in more
detail by way of Examples and Comparative Examples. The present
embodiment is not limited to the Examples below as long as they do
not go beyond the gist of the embodiment. The physical properties
of the Examples were measured by the methods below.
[0101] (1) Weight Average Molecular Weight
[0102] A calibration curve was created by performing measurements
using standard polystyrene under the following conditions using
ALC/GPC 150C Type.TM. produced by Waters Corporation.
[0103] Column: two GMH.sub.6-HT.TM.+two GMH.sub.6-HTL.TM. produced
by Tosoh Corporation,
[0104] Mobile phase: o-dichlorobenzene
[0105] Detector: differential refractometer
[0106] Flow rate: 1.0 ml/min
[0107] Column temperature: 140.degree. C.
[0108] Sample concentration: 0.1 wt %
(Weight Average Molecular Weight of Polyethylene)
[0109] The value for respective molecular weight on the obtained
calibration curve were multiplied by 0.43 (Q factor of
polyethylene/Q factor of polystyrene=17.7/41.3), whereby a
molecular-weight distribution curve was obtained in terms of
polyethylene, thus calculating the weight average molecular
weight.
(Weight Average Molecular Weight of Polypropylene)
[0110] Except for using 0.63 instead of 0.43, the weight average
molecular weight of polypropylene was calculated in the same manner
as the above.
(Weight Average Molecular Weight of Composition)
[0111] Except for using a Q-factor value for a polyolefin that
constitutes the largest mass fraction, the weight average molecular
weight of a composition was calculated in the same manner as the
case of polyethylene.
[0112] (2) Membrane Thickness (.mu.m)
[0113] The membrane thickness was measured at an ambient
temperature of 23.+-.2.degree. C. using a thickness micrometer,
KBN.TM. produced by Toyo Seiki Seisaku-sho, Ltd.
[0114] Thicknesses at five points located at substantially regular
intervals across the overall width in the TD direction were
measured, and an average of them was used as a representative
value.
[0115] (3) Porosity (%)
[0116] A sample of 10 cm by 10 cm square was cut out from a
microporous membrane, and its volume (cm.sup.3) and mass (g) were
measured. These values and the density (g/cm.sup.3) were used to
calculate the porosity from the formula below:
Porosity(%)=(volume-mass/mixture compound
density)/volume.times.100.
[0117] As the mixture compound density, a value calculated from the
densities and the mixture ratio of the raw materials used was
used.
[0118] (4) Air Permeability (sec/100 cm.sup.3)
[0119] The air permeability was measured with a Gurley densometer,
G-B2.TM. produced by Toyo Seiki Seisaku-sho, Ltd. in accordance
with JIS P-8117 (2009).
[0120] (5) Tensile Strength (MPa), Tensile Elastic Modulus
(N/cm)
[0121] MD and TD samples having a width of 10 mm and a length of
100 mm were used to measure these parameters with a tensile tester,
Autograph AG-A Type.TM. produced by Shimadzu Corporation in
accordance with JIS K 7127. A distance between chucks was adjusted
to 50 mm, and one of the surfaces of each of the end portions (25
mm) of a sample was taped with cellophane tape (N.29, product name;
product of Nitto Denko Packing System). In order to prevent slip of
the sample during the test, a fluoro rubber having a thickness of 1
mm was applied to the inside of the chuck of the tensile tester.
The stretching rate during the test was 200 mm/min.
[0122] The tensile strength (MPa) was determined by dividing the
strength at rupture by the cross-sectional area of the sample
before the test.
[0123] The tensile elastic modulus was determined from a gradient
of the stress-strain line for a segment where the elongation of the
sample is 1 to 4%. More specifically, in the stress-strain curve
obtained by plotting a relationship between the tensile strength
(MPa) applied to the sample in determining the above mentioned
tensile strength and the elongation of the sample (strain) (%), in
coordinate with the vertical axis indicating the tensile stress and
the horizontal axis indicating the elongation, a gradient of the
straight line (straight line portion) between two points of
elongation (strain)=1% and 4% was multiplied by the initial
thickness of the sample, whereby the tensile elastic modulus (N/cm)
was determined.
[0124] Herein, the elongation of the sample (%)=(the length after
stress applied-the length before stress applied)/the length before
stress applied.times.100.
[0125] (6) Outer Surface Roughness, Root Mean Square Value
Roughness And Average Length of Roughness Curvilinear Elements (Ra,
Rq, Sm) of Core
[0126] The surface roughness of the outer surface of a core (Ra
(arithmetic average roughness), Rq (root mean square value
roughness) and Sm (average length of roughness curvilinear
elements) were measured with Handysurf E-35A.TM. produced by Tokyo
Seimitsu Co., Ltd. The stylus tip was 90.degree. diamond cone with
5 .mu.mR, and the measurement was performed under the conditions of
evaluation length of 5 mm, evaluation speed of 0.6 mm/s, cutoff
value of 0.80 mm, and load of 4 mN or less. The measurement was
performed for the overall length of the core in the width direction
while setting the standard length at 5 mm to determine a minimum
value.
[0127] (7) Thermal Expansion Coefficient of Core (K.sup.-1)
[0128] A core was left to stand in each of the ovens adjusted at
298 K (25.degree. C.), 313 K (40.degree. C.), 333 K (60.degree. C.)
and 353 K (80.degree. C.) for 30 minutes, and immediately after
taking them out from the ovens (immediately after means within 30
seconds), the core outer diameter was measured with a dial
gauge.
[0129] Based on a temperature-expansion rate line, obtained by
plotting the measured values in a coordinate with a horizonal axis
indicating absolute temperatures and a vertical axis indicating
swelling rates ((the length the core after having left to stand at
the respective temperature for 30 minutes-the length of the core
after having left to stand at 25.degree. C.)/the length of the core
after having left to stand at 25.degree. C.), an approximate line
was drawn, and the thermal expansion coefficient (K.sup.-1), which
is a gradient of the approximate line and that is a swelling rate
per absolute temperature (K), was determined.
[0130] In order to determine the outer diameter when the core was
left to stand at 25.degree. C., diameters of the side face of the
core perpendicular to the rotating shaft at any measurement
position (measurement direction) and a position displaced from said
measurement position by 90.degree. (measurement direction
perpendicular to said measurement direction) were measured, and an
average value of them was used. The same points where the
measurements of the outer diameter when the core was left to stand
at 25.degree. C. were performed were used for measurements of the
outer diameters when the core was left to stand at 40.degree. C.,
60.degree. C., and 80.degree. C.
[0131] (8) Swelling Rate of Core for Humidity (%)
[0132] A core was left to stand at a temperature of 25.degree. C.
and under relative humidity of 50% for 24 hours, and the core
diameter was measured with a dial gauge.
[0133] Next, the core was left to stand at a temperature of
25.degree. C. and under relative humidity of 100% for 24 hours, the
core diameter was measured in the same way with a dial gauge, and
the swelling rate was calculated by the formula below:
Swelling rate(%)=(Core outer diameter under relative humidity of
100%-Core outer diameter under relative humidity of 50%)/(Core
outer diameter under relative humidity of 50%).times.100.
[0134] The outer diameter was determined in the same manner as
(7).
[0135] (9) Backlash of Core
[0136] As illustrated in FIG. 1, a core (a) was placed on a surface
plate (b) made of granite that is horizontally installed in such a
manner that side faces of the core perpendicular to the core
rotating shaft face upward and downward (so that one of the side
faces perpendicular to the core rotating shaft comes into contact
with the surface plate), and a dial gauge (c) produced by Ozaki
MFG. Co., LTD, 107-HG.TM. was set at any position of 2 mm inside
from the outermost circumference of the core, and a point where a
gauge head and the core just come into contact was set as a
reference point. Then, a displacement of the dial gauge when load
of 1 kg was applied to a position on the outermost circumference of
the core that is point-symmetrical position to the reference point
with respect to the center of the outermost circumference was
determined as the backlash.
[0137] Herein, the gauge head used was a needle gauge head produced
by Ozaki MFG. Co., LTD, XB-800.TM.. The measurement was performed
at 16 points for each of the both side faces of the core on the
outermost circumference at intervals of about 22.5.degree. as the
central angle, and the maximum value among the obtained backlash
values was determined as the maximum backlash of the core.
[0138] (10) The Number of Laminations/Winding Length
[0139] The number of laminations (times) of a microporous membrane
winding was measured, and the measured value was divided by the
winding length of the wound microporous membrane.
[0140] (11) Static Electricity (kV)
[0141] Static electricity of the outermost layer of the winding
immediately after winding of the microporous membrane (immediately
after means within 30 seconds) was measured with ASPURE static
meter produced by ASONE Corporation: YC102.TM..
[0142] (12) Battery Capacity Variation (%), Battery Heat Stability
(min.), and Battery Winding Property Evaluation
a. Preparation of Positive Electrode
[0143] 92.2% by mass of lithium-cobalt composite oxide LiCoO.sub.2
as a positive electrode active material, 2.3% by mass each of flake
graphite and acetylene black as electrically conductive agents, and
3.2% by mass of polyvinylidene fluoride (PVDF) as a binder were
dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This
slurry was applied onto one side of a 20 .mu.m-thick aluminum foil
serving as a positive electrode collector using a die coater, was
dried at 130.degree. C. for 3 minutes, and was compression-molded
with a roll press. At this time, coating was performed so that the
amount of positive electrode active material applied was 250
g/m.sup.2 and the bulk density of the active material was 3.00
g/cm.sup.3.
b. Preparation of Negative Electrode
[0144] 96.9% by mass of artificial graphite as a negative electrode
active material, 1.4% by mass of ammonium salt of
carboxymethylcellulose and 1.7% by mass of styrene-butadiene
copolymer latex as binders were dispersed in purified water to
prepare slurry. This slurry was applied onto one side of a 12
.mu.m-thick copper foil serving as a negative electrode collector
using a die coater, was dried at 120.degree. C. for 3 minutes, and
was compression-molded with a roll press. At this time, the amount
of negative electrode active material applied was 106 g/m.sup.2,
and the bulk density of the active material was 1.35
g/cm.sup.3.
c. Preparation of Nonaqueous Electrolyte Solution
[0145] LiPF.sub.6 as a solute was dissolved in a mixed solvent of
ethylene carbonate and ethyl methyl carbonate at a mixing ratio of
1 to 2 (v/v) so that the concentration was 1.0 mol/L, to prepare a
nonaqueous electrolyte solution.
d. Battery Assembly
[0146] A separator was cut into a circle of 18 mm.PHI. and a
positive electrode and a negative electrode were cut into a circle
of 16 mm.PHI., and they were stacked in the order of the positive
electrode, the separator, and the negative electrode so that the
active materials of the positive electrode and the negative
electrode face each other, which then was put in a stainless-steel
metal container with a lid. The container and the lid are insulated
from each other, where the container was in contact with the copper
foil as the negative electrode and the lid was in contact with the
aluminum foil as the positive electrode. This container was filled
with the above-mentioned nonaqueous electrolyte solution, and was
hermetically sealed. After leaving to stand at a room temperature
for one day, the battery was charged at a current value of 3 mA
(0.5 C) under the ambient of 25.degree. C. up to the battery
voltage of 4.2 V, and after reaching the voltage, reduction of the
current was started from 3 mA while keeping the voltage at 4.2 V,
thus performing the initial charging for 6 hours in total after the
preparation of the battery. Subsequently, the voltage was
discharged at the current value of 3 mA (0.5 C) to the battery
voltage of 3.0 V.
e. Battery Capacity Variation (%)
[0147] A battery was charged at a current value of 6 mA (1.0 C)
under the ambient of 25.degree. C. up to the battery voltage of 4.2
V, and after reaching the voltage, reduction of the current was
started from 6 mA while keeping the voltage at 4.2 V, thus
performing charging for 3 hours in total. Subsequently, the voltage
was discharged at the current value of 6 mA (1.0 C) to the battery
voltage of 3.0 V. The voltage capacity in this state was measured.
This operation was performed for 100 cells of batteries in total,
and the percentage of cells with variations of .+-.5% or greater
from the average capacity of the 100 cells was calculated, to
obtain a battery capacity variation.
Battery capacity variation(%)={the number of cells with a variation
beyond.+-.5% of the average capacity/100}.times.100(%).
f. Battery Heat Safety (min.)
[0148] The cells with a variation within .+-.5% of the average
capacity in the step e (cells free from variations) was heated from
a room temperature to 150.degree. C. at a rate of temperature
increase of 5.degree. C./min. After reaching 150.degree. C., the
temperature was maintained at 150.degree. C., and a time period
before the cells generate heat due to short-circuit was measured.
The heat generation was observed with a thermocouple connected with
a cell, and a time period required until the temperature reaches
155.degree. C. or greater was measured.
g. Battery Winding Property Evaluation
[0149] The mold obtained in the above step a was slit into the
width of 57.0 mm to obtain a positive electrode.
[0150] The mold obtained in the above step b was slit into the
width of 58.5 mm to obtain a negative electrode.
[0151] The positive electrode, a microporous membrane unwound from
a microporous membrane winding of Examples and Comparative
examples, the negative electrode and said microporous membrane were
stacked in this order, and then a wound electrode body was
manufactured in a conventional manner. Herein, the number of
windings was adjusted depending on the thickness of the microporous
membrane. An outermost end of the obtained wound electrode body was
fixed by taping with insulation tape. A negative electrode lead was
welded to a battery can, and a positive electrode lead was welded
to a safety valve, and the wound electrode body was placed in the
battery can.
[0152] This operation was repeated 100 times, and the number of
wound electrode bodies having a winding defect of misalignment or
wrinkles of the microporous membrane generated in placing a wound
electrode body in the battery can was counted, and the battery
winding property evaluation (%) was obtained by the formula
below:
Battery winding property evaluation(%)=100-winding defect
rate(%).
Example 1
[0153] 99% by mass of polyethylene homopolymer (PE(A)) with weight
average molecular weight of 1,000,000 and 1% by mass of
pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]
as an antioxidant were dry blended again with a tumbler blender, to
obtain a polymer-containing mixture. After the atmosphere was
substituted with nitrogen, the polymer-containing mixture thus
obtained was supplied to a twin-screw extruder in a nitrogen
atmosphere using a feeder. Liquid paraffin (with kinematic
viscosity of 7.59.times.10.sup.-5 m.sup.2/s at 37.78.degree. C.)
was injected into the cylinder of the extruder via a plunger
pump.
[0154] The feeder and the pump were adjusted so that a liquid
paraffin amount ratio in the total mixture to be extruded after
melting and kneading became 65% by mass (i.e., the polymer
concentration became 35% by mass). The melting and kneading were
performed under the conditions of a preset temperature of
200.degree. C., a screw rotation rate of 240 rpm, and a discharge
rate of 12 kg/h.
[0155] The kneaded melt was then extruded and cast through a T-die
onto a cooling roll controlled to a surface temperature of
25.degree. C., whereby a gel sheet with an original membrane
thickness of 1,400 .mu.m was obtained.
[0156] The gel sheet was then introduced into a simultaneous
biaxial tenter stretching machine and biaxially stretched. The
stretching was performed under the conditions of an MD draw
magnification of 7.0, a TD draw magnification of 7.0 (i.e.,
7.times.7 times) and a biaxial stretching temperature of
125.degree. C.
[0157] The resulting gel sheet was then introduced into a methyl
ethyl ketone tank, and was immersed completely in methyl ethyl
ketone to remove the liquid paraffin by extraction. Thereafter, the
methyl ethyl ketone was removed by drying.
[0158] The sheet was then introduced to a TD tenter for heat
setting (this may be abbreviated as "HS"). HS was performed at a
heat setting temperature of 125.degree. C. and of the stretch
magnification of 1.2 times, followed by relaxation operation of 0.8
times (i.e., HS relaxation ratio of 0.8 times).
[0159] Thereafter, the obtained microporous membrane underwent slit
processing into the width of 60 mm and the length of 50 m, and was
wound around a core of 65 mm in width (manufactured by coating the
surface of a paper core with acrylic resin, attaching a cutting
blade to the surface of the core while rotating the core, and
sweeping the surface in the width direction to cut the surface for
smoothing. After smoothing, the outer diameter of the core was 5
inches. Further, side faces of the core perpendicular to the
rotating shaft were polished with a surface polisher for
smoothing), thus obtaining a microporous membrane winding.
[0160] For a microporous membrane unwound from the winding,
physical properties, battery capacity variations, and battery heat
safety were evaluated. In the battery capacity variation
evaluation, 100 points located at intervals of 50 cm in the
lengthwise direction of the winding with winding length of 50 m
were used. Table 1 shows the result.
Example 2
[0161] In the same manner as in Example 1 except that a core used
had the properties shown in Table 1, a microporous membrane and a
microporous membrane winding were manufactured.
[0162] The outer diameter of the core was adjusted by adjusting the
thickness of a resin coat layer and the amount of cutting.
Example 3
[0163] In the same manner as in Example 2 except that the
microporous membrane subjected to heat setting and relaxation
operation was further shrunk by 2% at a temperature of 100.degree.
C. with a tenter in both of MD and TD directions to relax the MD
and TD tensile elastic moduli, a microporous membrane and a
microporous membrane winding were manufactured.
Example 4
[0164] In the same manner as in Example 3 except that a mixture of
50 parts by mass of PE(A) and 50 parts by mass of polyethylene
homopolymer (PE(B)) with weight average molecular weight of 300,000
were used instead of the polyethylene homopolymer (PE(A)) with
weight average molecular weight of 1,000,000, a microporous
membrane and a microporous membrane winding were manufactured.
Example 5
[0165] In the same manner as in Example 2 except that PE(B) was
used instead of PE(A), a microporous membrane and a microporous
membrane winding were manufactured.
Example 6
[0166] In the same manner as in Example 5 except that the same
shrinking process as in Example 3 was performed, a microporous
membrane and a microporous membrane winding were manufactured.
Example 7
[0167] In the same manner as in Example 5 except that a core used
was changed to that with the properties shown in Table 1 and the
microporous membrane subjected to heat setting and relaxation
operation was further stretched by 2% at a temperature of
100.degree. C. with a tenter in both of MD and TD directions so as
to adjust the tensile elastic modulus, a microporous membrane
winding was manufactured.
[0168] The properties of the core outer surface were adjusted by
adjusting the amount of cutting.
Examples 8 to 10
[0169] In the same manner as in Example 7 except that a core used
had the properties shown in Table 1, a microporous membrane and a
microporous membrane winding were manufactured.
[0170] As the core, a core made of acrylonitrile-butadiene-styrene
copolymer (ABS resin) was prepared, a cutting blade was attached to
the surface of the core while rotating the core, and the surface of
the core was swept in the width direction to cut the surface for
smoothing. The outer diameter of the core was adjusted by adjusting
the outer diameter of the prepared core made of ABS resin, and the
properties of the outer surface of the core were adjusted by
adjusting the cutting amount. Further, side faces of the core
perpendicular to the rotating shaft were polished with a surface
polisher for smoothing.
Example 11
[0171] In the same manner as in Example 10 except that an original
membrane thickness was changed to 900 .mu.m, a microporous membrane
and a microporous membrane winding were manufactured.
Example 12
[0172] In the same manner as in Example 11 except that the
microporous membrane subjected to heat setting and relaxation
operation was further shrunk by 2% at a temperature of 100.degree.
C. with a tenter in both of MD and TD directions to relax the MD
and TD elastic tensile moduli, a microporous membrane and a
microporous membrane winding were manufactured.
Example 13
[0173] In the same manner as in Example 12 except that a mixture of
95 parts by mass of PE(B) and 5 parts by mass of polypropylene
homopolymer (PP) with weight average molecular weight of 300,000
were used instead of PE(B), a microporous membrane and a
microporous membrane winding were manufactured.
Examples 14 to 20, 27
[0174] In the same manner as in Example 13 except that a core used
was changed to that with the properties shown in Table 2, a
microporous membrane and a microporous membrane winding were
manufactured.
Examples 21, 22
[0175] In the same manner as in Example 13 except that a core used
was changed to that with the properties shown in Table 2, a
microporous membrane and a microporous membrane winding were
manufactured.
[0176] The thermal expansion coefficient of the core was adjusted
by changing the copolymerization ratio of acrylonitrile, butadiene
and styrene of the ABS resin as core raw materials.
Examples 23 to 25
[0177] In the same manner as in Example 13 except that a core used
was changed to that with the properties shown in Table 2, a
microporous membrane and a microporous membrane winding were
manufactured.
[0178] As the core, a core made of Bakelite was prepared, a cutting
blade was attached to the surface of the core while rotating the
core, and the surface of the core was swept in the width direction
to cut the surface for smoothing. Further, side faces of the core
perpendicular to the rotating shaft were polished with a surface
polisher for smoothing.
[0179] The swelling rate of the core was adjusted by leaving the
core to stand at a high temperature for aging. In Example 25, the
side faces of the core perpendicular to the rotating shaft were not
polished with a surface polisher.
Example 26
[0180] In the same manner as in Example 24 except that the width of
the microporous membrane was slit into 150 mm, and was wound around
a core of 155 m in width, and a positive electrode slit into 147 mm
and a negative electrode slit into 148.5 mm were used in the
battery winding property evaluation, a microporous membrane and a
microporous membrane winding were manufactured.
Comparative Examples 1 to 4, 6
[0181] In the same manner as in Example 7 except that a core used
was changed to that with the properties shown in Table 3, a
microporous membrane and a microporous membrane winding were
manufactured.
[0182] As for the cores in Comparative Examples 1 and 2, surface
cutting was not performed after coating with resin.
[0183] As for the core in Comparative Example 6, surface cutting
was not performed.
Comparative Example 5
[0184] In the same manner as in Example 11 except that a core used
was changed to that with the properties shown in Table 3, a
microporous membrane winding was manufactured.
Comparative Example 7
[0185] In the same manner as in Example 26 except that a core used
was changed to that with the properties shown in Table 3 and
polyolefin used was changed to that shown in Table 3, a microporous
membrane and a microporous membrane winding were manufactured.
[0186] With respect to the microporous membrane unwound from the
above-stated microporous membrane windings in Examples 1 to 27 and
Comparative Examples 1 to 7, physical properties, battery capacity
variations, battery heat safety, battery winding properties, and
static electricity were evaluated. Tables 1 to 3 show the
result.
TABLE-US-00001 TABLE 1 Example 1 2 3 4 5 6 7 microporous PE(A) with
Mw of 1,000,000 (parts by 100 50 0 membrane raw mass) material
PE(B) with Mw of 300,000 (parts by 0 50 100 mass) PP with Mw of
300,000 (parts by 0 mass) microporous thickness (.mu.m) 14 membrane
porosity (%) 40 physical air permeability (sec/100 cm.sup.3) 200
properties at MD tensile strength (MPa) 140 outermost layer MD
tensile strength (MPa) 130 of microporous MD tensile elastic
modulus (N/cm) 140 115 105 95 80 115 membrane TD tensile elastic
modulus (N/cm) 130 110 100 80 60 110 winding static electricity at
outermost layer of microporous 0.9 0.7 0.7 0.7 0.7 0.7 0.5 membrane
winding (kV) core properties core outer diameter (inch) 5 6 8 core
material Paper core outer surface 2.6 2.6 2.6 2.6 2.6 2.6 1.8
arithmetic average roughness Ra (.mu.m) core outer surface 2.8 2.8
2.7 2.4 2.8 2.5 1.9 root mean square value roughness Rq (.mu.m)
core outer surface 225 225 225 225 225 225 190 average length of
roughness curvilinear elements Sm (.mu.m) core thermal expansion
coefficient -85 (.times.10.sup.-6/K) core swelling rate (%) 0.082
0.081 0.079 0.082 0.080 0.078 0.083 number of laminations (times)/
2.50 2.08 2.08 2.08 2.08 2.08 1.58 winding length (m) core backlash
0.08 0.07 0.08 0.08 0.07 0.07 0.06 microporous thickness difference
between 1.50 1.10 1.30 1.30 1.40 1.40 0.90 membrane outermost layer
and innermost layer physical of winding properties at porosity of
microporous membrane at 32 35 34 34 33 33 36 innermost layer
innermost layer of winding (%) of microporous air permeability of
microporous 270 240 250 250 260 260 230 membrane membrane at
innermost layer of winding winding (sec/100 cm.sup.3) battery
battery capacity variation (%) 9 5 6 6 5 6 3 evaluation battery
heat safety (min.) 13 13 18 25 33 42 30 battery winding property
(%) 91 93 93 92 93 93 95 Example 8 9 10 11 12 microporous PE(A)
with Mw of 1,000,000 (parts by 0 membrane raw mass) material PE(B)
with Mw of 300,000 (parts by 100 mass) PP with Mw of 300,000 (parts
by 0 mass) microporous thickness (.mu.m) 14 9 membrane porosity (%)
40 physical air permeability (sec/100 cm.sup.3) 200 130 properties
at MD tensile strength (MPa) 140 outermost layer MD tensile
strength (MPa) 130 of microporous MD tensile elastic modulus (N/cm)
115 90 membrane TD tensile elastic modulus (N/cm) 110 80 winding
static electricity at outermost layer of microporous 0.5 0.5 0.5
0.5 0.5 membrane winding (kV) core properties core outer diameter
(inch) 8 core material ABS core outer surface 0.9 0.4 0.2 0.2 0.2
arithmetic average roughness Ra (.mu.m) core outer surface 1.0 0.5
0.3 0.2 0.3 root mean square value roughness Rq (.mu.m) core outer
surface 190 160 145 145 145 average length of roughness curvilinear
elements Sm (.mu.m) core thermal expansion coefficient 90
(.times.10.sup.-6/K) core swelling rate (%) 0.032 0.031 0.033 0.033
0.032 number of laminations (times)/ 1.58 1.58 1.58 1.59 1.59
winding length (m) core backlash 0.05 0.04 0.03 0.03 0.03
microporous thickness difference between 0.40 0.20 0.10 0.10 0.20
membrane outermost layer and innermost layer physical of winding
properties at porosity of microporous membrane at 38 39 40 40 39
innermost layer innermost layer of winding (%) of microporous air
permeability of microporous 215 210 200 130 130 membrane membrane
at innermost layer of winding winding (sec/100 cm.sup.3) battery
battery capacity variation (%) 2 1 0 3 4 evaluation battery heat
safety (min.) 31 32 31 29 38 battery winding property (%) 95 95 94
94 94
TABLE-US-00002 TABLE 2 Example 13 14 15 16 17 18 19 20 microporous
PE(A) with Mw of 1,000,000 (parts by 0 membrane raw mass) material
PE(B) with Mw of 300,000 (parts by 95 mass) PP with Mw of 300,000
(parts by 5 mass) microporous thickness (.mu.m) 9 membrane porosity
(%) 40 physical air permeability (sec/100 cm.sup.3) 130 properties
at MD tensile strength (MPa) 140 outermost layer MD tensile
strength (MPa) 130 of microporous MD tensile elastic modulus (N/cm)
90 membrane TD tensile elastic modulus (N/cm) 80 winding static
electricity at outermost layer of microporous 0.5 0.3 0.2 0.2 0.2
0.2 0.2 0.2 membrane winding (kV) core properties core outer
diameter (inch) 8 9 10 core material ABS core outer surface 0.2
arithmetic average roughness Ra (.mu.m) core outer surface 0.2 0.2
1.6 1.1 1.1 1.1 1.1 0.2 root mean square value roughness Rq (.mu.m)
core outer surface 145 90 90 90 220 280 320 90 average length of
roughness curvilinear elements Sm (.mu.m) core thermal expansion
coefficient 90 (.times.10.sup.-6/K) core swelling rate (%) 0.031
0.031 0.030 0.035 0.034 0.031 0.032 0.032 number of laminations
(times)/ 1.59 1.41 1.25 1.25 1.25 1.25 1.25 1.25 winding length (m)
core backlash 0.04 0.03 0.04 0.02 0.04 0.05 0.04 0.03 microporous
thickness difference between 0.20 0.15 0.70 0.40 0.60 0.80 1.20
0.10 membrane outermost layer and innermost layer physical of
winding properties at porosity of microporous membrane at 39 39 34
38 35 33 31 40 innermost layer innermost layer of winding (%) of
microporous air permeability of microporous 130 130 170 140 160 170
190 130 membrane membrane at innermost layer of winding winding
(sec/100 cm.sup.3) battery battery capacity variation (%) 4 3 7 4 7
8 9 2 evaluation battery heat safety (min.) 47 47 47 47 44 46 45 47
battery winding property (%) 95 96 99 99 97 98 99 98 Example 21 22
23 24 25 26 27 microporous PE(A) with Mw of 1,000,000 (parts by 0
membrane raw mass) material PE(B) with Mw of 300,000 (parts by 95
mass) PP with Mw of 300,000 (parts by 5 mass) microporous thickness
(.mu.m) 9 membrane porosity (%) 40 physical air permeability
(sec/100 cm.sup.3) 130 properties at MD tensile strength (MPa) 140
outermost layer MD tensile strength (MPa) 130 of microporous MD
tensile elastic modulus (N/cm) 90 membrane TD tensile elastic
modulus (N/cm) 80 winding static electricity at outermost layer of
microporous 0.2 0.2 0.2 0.2 0.2 0.2 0 membrane winding (kV) core
properties core outer diameter (inch) 10 15 core material ABS
Bakelite ABS core outer surface 0.2 arithmetic average roughness Ra
(.mu.m) core outer surface 0.2 0.2 0.2 0.2 0.2 0.2 0.2 root mean
square value roughness Rq (.mu.m) core outer surface 90 90 90 90 90
90 90 average length of roughness curvilinear elements Sm (.mu.m)
core thermal expansion coefficient 120 160 45 45 45 45 90
(.times.10.sup.-6/K) core swelling rate (%) 0.035 0.030 0.031 0.020
0.020 0.020 0.031 number of laminations (times)/ 1.25 1.25 1.25
1.25 1.25 1.25 0.84 winding length (m) core backlash 0.04 0.03 0.05
0.04 0.32 0.04 0.03 microporous thickness difference between 0.20
0.50 0.10 0.00 0.00 0.00 0.00 membrane outermost layer and
innermost layer physical of winding properties at porosity of
microporous membrane at 39 36 40 40 40 40 40 innermost layer
innermost layer of winding (%) of microporous air permeability of
microporous 130 150 130 130 130 130 130 membrane membrane at
innermost layer of winding winding (sec/100 cm.sup.3) battery
battery capacity variation (%) 4 7 1 0 0 0 1 evaluation battery
heat safety (min.) 44 45 46 46 47 47 46 battery winding property
(%) 98 98 99 98 92 96 100
TABLE-US-00003 TABLE 3 Comparative Example 1 2 3 4 5 6 7
microporous PE(A) with Mw of 1,000,000 (parts by 0 membrane raw
mass) material PE(B) with Mw of 300,000 (parts by 100 mass) PP with
Mw of 300,000 (parts by 0 mass) microporous thickness (.mu.m) 14 9
14 9 membrane porosity (%) 40 physical air permeability (sec/100
cm.sup.3) 200 130 200 130 properties at MD tensile strength (MPa)
140 outermost layer MD tensile strength (MPa) 130 of microporous MD
tensile elastic modulus (N/cm) 115 90 membrane TD tensile elastic
modulus (N/cm) 110 80 winding static electricity at outermost layer
of microporous 0.4 1.2 1.2 1.2 1.2 0.5 1.2 membrane winding (kV)
core properties core outer diameter (inch) 8 4 8 4 core material
Paper ABS core outer surface 3.6 3.6 1.4 0.3 0.3 3.6 0.2 arithmetic
average roughness Ra (.mu.m) core outer surface 3.8 3.6 1.5 0.4 0.3
3.8 0.3 root mean square value roughness Rq (.mu.m) core outer
surface 225 225 145 145 145 225 145 average length of roughness
curvilinear elements Sm (.mu.m) core thermal expansion coefficient
-85 -85 90 (.times.10.sup.-6/K) core swelling rate (%) 0.081 0.083
0.032 0.031 0.030 0.032 0.032 number of laminations (times)/ 1.58
3.12 3.12 3.12 3.14 1.58 3.14 winding length (m) core backlash 0.08
0.09 0.05 0.06 0.05 0.03 0.05 microporous thickness difference
between 2.60 2.90 2.60 2.40 1.90 2.50 1.90 membrane outermost layer
and innermost layer physical of winding properties at porosity of
microporous membrane at 26 24 26 28 24 27 24 innermost layer
innermost layer of winding (%) of microporous air permeability of
microporous 340 360 340 320 220 340 220 membrane membrane at
innermost layer of winding winding (sec/100 cm.sup.3) battery
battery capacity variation (%) 14 17 14 11 13 11 12 evaluation
battery heat safety (min.) 31 31 32 31 29 30 29 battery winding
property (%) 98 89 88 89 90 98 82
[0187] A microporous membrane obtained from a microporous membrane
winding of the present invention is favorably used as a separator
for a high-capacity lithium-ion secondary battery using a
particularly thin membrane
* * * * *